1. Technical Field
A magnetic random access memory (abbreviated as xe2x80x98MRAMxe2x80x99) is disclosed. More specifically, an improved MRAM having a higher speed than an SRAM, integration density as high as a DRAM, and the properties of a nonvolatile memory such as a flash memory, is disclosed
2. Description of the Related Art
Most of the semiconductor memory manufacturing companies have developed the MRAM which uses a ferromagnetic material as one of the next generation memory devices.
The MRAM is a memory device for reading and writing information by forming multi-layer ferromagnetic thin films, and sensing current variations according to a magnetization direction of the respective thin films. The MRAM has high speed, low power consumption and high integration density due to the special properties of the magnetic thin film, and performs a nonvolatile memory operation such as a flash memory.
In its function as a memory device, the MRAM utilizes a giant magneto resistive (GMR) or spin-polarized magneto-transmission (SPMT) phenomenon, which is generated when the spin influences electron transmission.
The MRAMs using GMR phenomenon utilize a phenomenon in which resistance is remarkably varied when spin directions are different in two magnetic layers having a non-magnetic layer disposed between the two magnetic layers.
The MRAMs using SPMT utilize a phenomenon in which larger current transmission is generated when spin directions are identical in two magnetic layers having an insulating layer disposed therebetween.
MRAM research, however, is still in its early stages, and is concentrated mostly on the formation of multi-layer magnetic thin films and less on the research of unit cell structure and peripheral sensing circuits.
FIGS. 1a and 1b are a cross-sectional view and a layout view, respectively, of a conventional MRAM. One bit line and one word line are formed in pairs for each magnetic tunnel junction (MTJ) cell, which makes it difficult to obtain spacing between metal wires. Here, FIG. 1a is a cross-sectional view taken along line Axe2x80x94A of FIG. 1b. 
Referring to FIG. 1a, the conventional MRAM includes first word lines 13, which are a pair of gates formed on a semiconductor substrate 11. A first impurity junction region 15-1 and a pair of second impurity junction regions 15-2 are also formed on the semiconductor substrate 11. The first impurity junction region 15-1 is disposed between the pair of first word lines 13, and the second impurity junction regions 15-2 are disposed on both sides of the first impurity junction region 15-1 so that the pair of first word lines 13 lies on the semiconductor substrate 11 between the first impurity junction region 15-1 and the second impurity junction regions 15-2. A ground line 23 is connected to the first impurity junction region 15-1 through a contact plug 19. A pair of connection layers 27 is connected to the pair of second impurity junction regions 15-2 through a stacked structure of a first contact plug 17, a conductive layer 21, and a second contact plug 24. Second word lines 25 are formed above the first word lines 13 and disposed below the connection layers 27. The pair of MTJ cells 29 is formed on the connection layers 27 that are disposed above the second word line 25, and has a width as large as that of the second word lines 25. Bit lines 33 are connected to the MTJ cells 29 through third contact plugs 31 that are vertical to the first and the second word lines 13 and 25. The ground line 23 is formed at the center portion, and the first word lines 13, the connection layers 27, the second word lines 25, and the MTJ cells 29 are symmetrically formed with respect to the ground line 23.
Referring to FIG. 1b, in area of one MRAM cell is 2Fxc3x976F, i.e., 12F2, where xe2x80x98Fxe2x80x99 is the minimum size of a line/space width that can be formed according to a lithography process.
As described above, the conventional MRAM has one bit line and word lines formed in pairs for each MTJ cell 29. This makes it difficult to obtain sufficient spacing for metal wires in order to achieve a high integration density of the device due to increased cell size.
A magnetic random access memory (MRAM) is disclosed, which achieves high integration by forming a second word line that serves as two write lines of MRAMs arranged vertically so that one line can apply a magnetic field to two MTJ cells.
There is provided an MRAM including a pair of first word lines formed on a semiconductor substrate; a first impurity junction region and a pair of second impurity junction regions formed on the semiconductor substrate, the first impurity junction region disposed between the pair of first word lines and the pair of second impurity junction regions disposed on both sides of the first impurity junction region so that the pair of first word lines lies on the semiconductor substrate between the first impurity junction region and the pair of second impurity junction regions; a ground line connected to the first impurity junction region; a pair of connection layers respectively connected to the pair of second impurity junction regions; a pair of MTJ cells respectively connected to the pair of connection layers; a pair of bit lines respectively connected to the pair of MTJ cells; a second word line, which is a write line, formed above the ground line to be electrically isolated from the ground line; and a metal wire connected to the second word line, the metal wire running in the perpendicular direction to the pair of bit lines, and wherein each bit line has a thickness ranging from 4000 to 5000 xc3x85. The distance between the MTJ cell and the metal wire ranges from 10000 to 50000 xc3x85. The distance between the bit line and the metal wire ranges from 1000 to 3000 xc3x85. The second word line is used as an outbound current path, the metal wire and second word line is connected by a metal wire contact plug, and the distance between the MTJ cell and the metal wire contact plug ranges from 0.5 F to 1.9 P.